Power supplies are typically used in powering gas discharge lighting, to convert a low impedance, low voltage power source, such as a 120 Volt 60 Hz AC wall outlet, into a higher voltage source suitable for connection. A transformer is used to step up the voltage of the 120 Volt AC source. The 120 Volt AC source is connected to the primary winding of the transformer and the secondary winding of the transformer is connected to the gas discharge lamp.
A neon sign (hereinafter also called "neon tubing"), is one example of a gas discharge lamp. Neon signs typically use a transformer (hereinafter also called a "neon transformer") to illuminate the sign. The following discussion of the background and the invention will refer to power supply circuits used for neon signs, however, it will be understood that principles of the present invention have application to power supply circuits for other gas discharge tube lamps as well.
A primary concern with known neon power supplies, is the potential that a ground fault from the high voltage outputs of the power supply can create substantial current flows, potentially causing fires if the ground fault creates an arc involving flammable materials. A potentially dangerous ground fault current may occur anytime there is a relatively low impedance path from one of the high voltage output leads of the neon power supply to ground. Such a path may be formed if a neon sign is carelessly installed so that one of the output leads connected to the sign is in contact with a low impedance in a window frame, doorway, or other ground-connected relatively low impedance.
To detect ground fault current, it is typically necessary to couple a ground fault detection circuit to the secondary winding of the power supply transformer, and/or to the neon sign itself. Specifically, the ground fault detection circuit may be coupled between a path to ground, and either a center tap of the secondary winding of the transformer, and/or a return point located near the electrical mid-point of the neon tubing. If there is a secondary ground fault, the transformer circuit automatically interrupts power.
Troubleshooting a neon sign for ground faults is difficult, because ground faults are not always visibly detectable. Often, it is necessary to carefully measure currents flowing through various connections to determine the location of the fault, which can be an arduous process. If difficulty in locating a fault may tempt the installer to conclude that the secondary ground fault interruption circuitry is malfunctioning, and try to defeat the ground fault circuitry.
Thus, there is a need for circuitry which enables a gas discharge tubing installer to identify and pinpoint the location of a ground fault quickly and accurately, to speed installation and minimize the temptation for tampering with the ground fault detection circuitry.
The circuitry described in the above referenced U.S. patent application Ser. No. 08/838,060 includes several features for preventing the ground fault detection circuitry from being inadvertently or deliberately defeated. A difficulty with this and other known secondary ground fault interrupting circuits, is that none can reliably detect a ground fault reliably when the center of the gas tube load is connected to earth ground. This is due to cancellation of the opposite and nearly equal currents entering the ground node. The net resulting current into/out of ground may not be sufficient to activate a secondary ground fault detection circuit connected between a midpoint of the transformer secondary and ground. Thus, the ground fault detection features of these circuits can often be defeated by grounding the mid-point of the load.
For example, FIG. 1 illustrates a neon transformer circuit 10 with a secondary ground fault interrupter, such as that described in the above-referenced U.S. patent application Ser. No. 08/838,060, connected to a load which is grounded at its mid-point. The neon transformer includes a primary winding 12 and secondary windings 14 and 16. The leads of primary winding 12 are connected to 120 Volt AC power via switch 13, causing secondary windings 14 and 16 to develop substantially higher voltages for driving the load. Secondary windings 14 and 16 are drawn as two windings connected in series at a common node 17, but could also be a single secondary winding with a center tap. The center tap or common node 17 of the secondary windings is connected through a ground fault current detection circuit 18 to a path to ground. If detection circuit 18 senses any substantial current flow between node 17 and ground, circuit 18 generates a SHUT DOWN signal on line 20, causing switch 13 to open and remove power from the transformer circuit.
As noted above, a secondary ground fault detector of this kind can be defeated by connecting a mid-point of the load to ground. As seen in FIG. 1, the load connected to secondary windings 14 and 16 includes two series-connected loads 22 and 24, as well as a ground fault path 26 from secondary 14 to ground.
Typically, the neon transformer is a leakage reactance type transformer which exhibits a relatively constant current over a wide range of load impedances. Furthermore secondary windings 14 and 16 are typically virtually identical and therefore produce similar load currents that are opposing (180 degrees out of phase).
Under these conditions, note that the current flowing out of secondary 14 divides and flows into two separate paths. Some current flows through load 22 into ground while the remainder flows through the ground fault path 26 into ground. The current then recombines and flows through ground, through the ground fault current detector 18 and back to secondary 14. The current from secondary 16 flows through the ground fault current detector 18 into ground, through load 24, and back to secondary 16. The current from secondary 16 that flows through the ground fault current detector 18 is similar in magnitude but opposite in direction to the current flowing from secondary 14. This results in little or no net current flow through the ground fault current detector 18, which may not activate despite the presence of a fault current through the fault path 26 which is sufficient to start a fire.
Accordingly, there is a need for a transformer circuit with secondary ground fault detection which cannot be defeated by shorting a mid-point of the load to ground.
An additional common difficulty with transformer circuits is momentary inrush current experienced when power is initially applied. This inrush current can be twenty times greater than the normal steady state operating current, and last ten to twenty AC cycles before normal steady state operating conditions are achieved. This large current can cause nuisance tripping of circuit breakers, and lead to premature fuse or circuit breaker failures. Line voltage sags resulting from high inrush currents can also interfere with other electronic equipment connected to the AC line.
Properly designed transformers, when connected to standard 120 Volt AC power, will achieve a steady state in which the magnetic field intensity and flux density vary with the line voltage within the linear region of the B-H curve of the core, with relatively low loss and low current. When power is removed, this variation will cease. However, if power is suddenly disconnected when the magnetic flux density in the core is near its peak value, a residual amount of magnetic flux density will remain in the core. The core will retain this residual magnetic flux until power is reapplied. The residual magnetic flux is not in itself harmful; however, when power is reapplied, if the initial half-cycle of the applied line voltage generates magnetic flux in the same direction as the residual flux in the core, at the first peak of the applied line voltage, the magnetic flux density in the core will substantially exceed the steady state peak flux density. This can drive the magnetic core of the transformer into the nonlinear (saturated) region of its B-H curve, where the primary inductance of the transformer decreases radically. At this point a very large inrush current will be drawn by the transformer. With successive cycles of the applied line voltage, for as long as ten to twenty cycles, the magnetic core will continue to be driven in one direction into the nonlinear region of the B-H curve of the core, with the extent of the excursion slowly decreasing until steady state operation is achieved. During these ten to twenty cycles, decreasing levels of inrush current will be drawn corresponding to each peak of the applied line voltage, leading to the problems identified above.
Thus, there is a need for a transformer circuit which exhibits reduced inrush currents when power is initially applied.